The requirements on exactness in regard of time and frequency for communicating over the air interface in communication systems using radio access technologies such as e.g. LTE (Long Term Evolution) or WCDMA (Wideband Code Division Multiple Access) are very high. For example, an LTE macro base station is required not to deviate in frequency from air interface-related frequencies more than 50 ppb (parts per billion), i.e. 50×10−9 Hz, and not to deviate in time more than 1.5 μs for LTE-TDD (Time Division Duplex).
In order to fulfill these strict time and frequency requirements for communicating over the air interface, a base station normally relies on an external source to provide frequency and time synchronization.
For a base station with IP backhaul, frequency and time synchronization could be obtained from an external time server connected to the IP network, by use of protocols like IEEE 1588 or NTP (Network Time Protocol). Another solution is to use a GPS (Global Positioning System) receiver to achieve time and frequency synchronization.
However, the current solutions described above have several drawbacks, especially when deployed for smaller indoor base stations, such as e.g. pico base stations and/or home base stations i.e. so-called femto products. For example, GPS receivers do not function well in unfavorable radio conditions, such as indoor environments, since they require line of sight to a number of satellites. Further, synchronization by use of protocols such as NTP and IEEE 1588 implies that very strict requirements must be fulfilled by the backhaul link, in regard of e.g. packet loss and jitter. Typically, many IP connections, e.g. in home environments, do not fulfill such high requirements.
Thus, the synchronization of small base stations, such as e.g. pico base stations and home indoor base stations, having unfavorable radio conditions, is identified as a problem.